EVIDENCE FOR A BI-VOLTINE LIFE CYCLE
IN CULISETA MELANURA

WAYNE J. CRANS AND FARIDA MAHMOOD

Mosquito Research & Control

Rutgers-The State University

180 Jones Avenue

New Brunswick, NJ 08901-8536

INTRODUCTION

Culiseta melanura is a bird feeding mosquito that serves as the
primary maintenance vector of eastern equine encephalitis virus(EEEV)
in the Atlantic flyway. The mosquito has an affinity for acid
water and develops as a larva in subterranean crypts (Orrel 1997).
Crans and McNelly (1997) used Cs. melanura as a model for a life
cycle type that lays non-desiccation resistant eggs directly on
water, develop as a larva in freshwater swamp habitats, have multiple
generations each year and overwinter in the larval stage. Mahmood
and Crans (1998) followed the life cycle of Cs. melanura under
controlled conditions in the laboratory and obtained evidence
to suggest that the species is probably bi-voltine, rather than
multi-voltine in most of the northeastern United States. This
paper summarizes the results of those studies and shows how bi-voltinism
may regulate EEEV activity each year.

RECRUDESCENCE OF INFECTIOUS VIRUS IN BIRDS

The overwintering mechanism(s) for EEEV remained an enigma for
many years. Crans et al. (1994) sampled birds and mosquitoes simultaneously
at a site in southern New Jersey and found that virus repeatedly
appeared in birds before it cycled in mosquitoes. They hypothesized
that EEEV probably overwintered as a latent infection in previously
infected resident birds and recrudesced in spring and early summer
during the stress of the nesting season. They also hypothesized
that an influx of newly emerged Cs. melanura was needed
in the presence of non-immune juvenile hosts to initiate amplification
in any geographic area. Their studies did not suggest a mechanism
that would explain the appearance of large numbers or nullipars
at any point in the season.

AVIAN NESTING IN THE NORTHEAST

In most of the northeastern United States, summer resident bird
species (Wood Thrush, Catbird) return from their wintering sites
in the south in late April and early May to join permanent resident
bird species (Cardinal, Chickadee, Tufted Titmouse) that remain
on site all winter long. These resident species nest during the
month of June and fledge their young by early July. The juveniles
remain on site in large numbers until September when migration
takes place. The summer residents then join large numbers of migrants
that fly south to their overwintering sites. At the same time,
the permanent resident bird species that remain on site are joined
by winter resident birds that nested and fledged their young further
north. Crans et al. (1994) showed that recrudescence of latent
virus took place as early as May or June, but amplification never
occurred until July at the earliest. This meant that there appeared
to be a 1-2 month lag in the cycle between appearance of virus
and amplification in juveniles. At no time, during their investigations,
did virus appear in Cs. melanura during the month of June.

EVIDENCE FOR A BI-VOLTINE LIFE CYCLE IN CS. MELANURA

Mahmood and Crans (1998) showed that Cs. melanura was a
mosquito with an exceptionally slow metabolism. Egg hatch to pupation
takes 1 month at 79o F, 3 months at 61o
F and 7 months at 50o F. The species had a thermal
minimum that approached 50o F but was able to survive
temperatures well below that range in a state of torpor. The mosquito
develops significantly slower than other species and appears well
suited to the cold water crypts that make up its primary breeding
habitat. Burbutis and Lake (1956) took water temperatures from
Cs. melanura crypts in New Jersey and found that they remained
below 50o F from November to April and rarely exceeded
60o F during the summer. Joseph and Bickley (1969)
made similar measurements in Maryland and showed variations of
only several degrees in that area to the south. Using the models
of Mahmood and Crans (1998), Cs. melanura would take 3
months to develop in those cold water habitats. Eggs laid from
adults that emerged from overwintering larvae in May would not
produce adults until August. Eggs laid in August would overwinter
as larvae and not emerge until the following spring. This would
allow only 2 generations per year and result in a bi-voltine life
cycle.

IMPACT OF A BI-VOLTINE LIFE CYCLE

Considering the structure of the avian nesting season in the northeast,
the 2 generations of Cs. melanura would encounter very
different bird populations. The spring generation of Cs. melanura
that emerge from overwintering larvae would emerge to feed on
adult birds that are in the process of setting up nesting territory.
Adult birds are poor amplification hosts because most already
possess immunity. As a result, the spring generation would encounter
recrudescing virus in the absence of susceptible amplification
hosts. The summer generation emerges after large numbers of juveniles
have joined the population. As a result, virus could race through
the population just before the fall migration takes place. If
the summer generation of Cs. melanura emerges late (cold
water temperatures in the crypts), it would encounter a migrating
bird population. This would negate virus amplification on site
by constantly moving the virus southward. As a result, temperature
appears to regulate the northern limits for virus amplification
each year. In geographic areas where 2nd generation
of Cs. melanura encounters large numbers of juvenile birds
that remain on site, amplification of virus takes place. In geographic
areas where the 2nd generation of Cs. melanura
encounters the fall migration, the virus cycle is essentially
aborted.

Further field data are needed to test the validity of the hypothesis
presented here. Studies are currently underway to make the comparisons
needed.

ACKNOWLEDGMENTS

This is New Jersey Agricultural Experiment Station Publication
No. E-40101-01-99 supported by state funds and funds from the
New Jersey State Mosquito Control Commission.

Mahmood, F. and W. J. Crans. 1998. Effect of temperature
on the development of Culiseta melanura (Diptera: Culicidae)
and its impact on amplification of eastern equine encephalomyelitis
virus in birds. J. Med. Entomol. 35:1007-1012.

Orrell, E. P. 1997.

The population dynamics of Culiseta melanura larvae at
an endemic focus for eastern equine encephalitis virus in a New
Jersey Coastal cedar swamp. Proc. Northeastern Mosq. Control Assoc.
43:38-39.